1. Technical Field
The invention relates generally to pressure relief systems and, more particularly, to a method and apparatus for relieving pressure in a pressurized space between two casings, or other members, in a well.
2. Background Art
Drilling of a well through subsurface formations typically involves progressively running casings into the well. Normally, the well is drilled to an initial depth and a conductor casing is run into the well and cemented to the well. A wellhead is typically mounted on the upper end of the conductor casing to provide means for suspending additional casings in the well. The rest of the well is drilled in sections with an intermediate casing run into the well after drilling of each section. The intermediate casings are concentrically arranged in the well with the innermost casing having the smallest diameter among all the casings and extending to a desired well depth, typically near a production zone. FIG. 1 shows a conductor casing 10 that is secured in a well 12 by a cement sheath 14. Intermediate casings 16 and an innermost or production casing 18 are suspended in the well 12. As shown, adjacent casings and the surrounding formation define annular spaces 20. The annular spaces 20 are sealed at the top by the wellhead 22 and closed at the bottom by the formation.
The well is drilled by lowering an appropriately sized drill bit on the end of a drill string into the well and operating the drill bit to cut the formation. While operating the drill bit, drilling fluid is pumped through the drill string to move the earth cuttings away from the bottom of the well to the surface. The drilling fluid in the well also serves to control formation fluid influx into the well. The casings are run into the well with the drilling fluid in the well so that the drilling fluid is trapped in the annular spaces 20 between the casings. Typically, cement is pumped into the annular spaces 20 to displace the drilling fluid, secure the casings to the well, and prevent formation fluid influx into the annular spaces 20. As can be appreciated, for casings extending several hundred feet into the well, substantial volumes of cement are required to fill the annular spaces. Thus, from an economic standpoint, it would be desirable to not displace the drilling fluid in the annular spaces with cement or to partially fill the annular spaces with cement, preferably the bottom portions of the annular spaces that are exposed to the surrounding formations.
The well is put to production after it is completed. Completion of the well may include suspending a liner 24 near the bottom end of the production casing 18. The liner 24 includes perforations through which formation fluid may enter the liner 24 and flow into a production tubing 26. A packer 28 isolates the section of the well to be produced by sealing an annular space between the production tubing 26 and the production casing 18. During production, if drilling fluid is trapped in any one of the annular spaces 20, the temperature of the drilling fluid trapped in the annular space rises to the temperature of the flowing formation fluids, resulting in expansion of the trapped drilling fluid. Because the annular space is closed, the pressure of the expanding drilling fluid also rises. When the pressure of the trapped drilling fluid exceeds the fracture pressure of the surrounding formation, the drilling fluid is forced into the formation adjacent the annular space and the pressure in the annular space stops rising. However, if the formation is plugged for some reason such that the fluid is unable to enter the formation, the fluid pressure in the annular space will continue to rise and may eventually cause the casings to burst or collapse. The formation may be plugged because it is cemented off. Even if the formation is not cemented off, the formation may still be plugged if the drilling fluid in the annular space is weighted with solids and the solids fall down and accumulate in the annular space as the temperature of the drilling fluid in the annular space rises.
It is undesirable to have the casings burst since this will lead to loss in control of the well. Thus, it has been the typical practice to completely fill the annular spaces with cement so that the pressure rise due to thermal expansion of trapped drilling fluid in the annular spaces is eliminated. However, for economical reasons, it is desirable to be able to produce the well with the annular spaces unfilled or partially filled with cement. One way of accomplishing this feat is to make the casings strong enough to withstand pressure increases that may occur due to thermal expansion of trapped drilling fluid in the annular spaces. This generally means heavier and more expensive casings, along with more expensive equipment for running the casings into the well, and may not result in cost savings over the typical practice of filling the annular spaces with cement.
Another method for preventing casings from bursting when the drilling fluid trapped in the annular spaces expands is to run a pressure relief valve between the casings. For example, a pressure relief valve may be run on the production casing such that fluid transfer from the annular spaces to the production casing occurs. The fluid in the production casing is maintained at a desired pressure and the pressure relief valve operates to equalize the pressure in the annular spaces with the pressure in the production casing. However, with the equalizing of pressure comes mixing of fluid in the annular spaces with the fluid in the production casing. This mixing of fluids, e.g., drilling fluid and completion fluid, may be undesirable. The drilling fluid in the annular spaces may contain solids which can accumulate in the production casing and settle on the packer 28. Also, if the pressure relief valve seal fails, a leak path is created between the casings, creating a potential for uncontrolled fluid transfer between the casings.